1. Fields of the Invention
The present invention relates to a self-regulating electric heater employing a resistor of positive temperature coefficient of resistivity (hereinafter referred to as PTC resistor) as a heating element, and more particularly to a self-regulating electric heater such as may be adapted to an oral inhalation device for producing steam which is to be inhaled in spray form together with suitable medicines carried on the steam.
2. Description of the Prior Art
Such PTC resistor has been found to be advantageous when used as heaters because of its self-regulating characteristic. However, the size of the PTC resistor is practically limited to be comparatively small so that the amount of heat dissipated therefrom is limited to a lower level. Therefore, there have been proposed for the purpose of increasing heat transfer efficiency many schemes to reduce as much as possible heat loss in transferring the heat derived from the PTC resistor to a casing to which it is attached together with electrodes supplying electricity to the PTC resistor and from which the heat is dissipated for heating purpose. Among the above schemes is to have the electrodes in pressing contact with the casing to provide a close and stable heat transfer relation therebetween as disposing the PTC resistor between the electrodes. Exemplary of the above is shown in U.S. Pat. No. 4,147,927 for a self-regulating heating element in which two electrically and thermally conductive flat strips or electrodes parallel to and spaced from each other are positioned in a longitudinally arranged tubular body with one or more PTC resistors between and in electrical and heat-exchange contact with the parallel flat strips. The flat strips are inserted in the tubular body to be in resilient contact with the inner surface of the body while the PTC resistors are fixed between the strips by respective layers of solder. However, such soldering connection between the PTC resistors and the strips should require a maximum contacting area therebetween for maintaining the heat transfer efficiency, which can be only achieved with a careful and skillful soldering technique, therefore rendering the assembly of the heater cumbersome. Also from the point of view that the solder is likely to melt at the operating temperature of the heater so as to render the PTC resistors no longer fixed on the strips, the PTC resistors are susceptible to possible mechanical shocks acted upon the heater such that they might be moved out of optimum electrical and heat transfer relation from the strips, rendering the heater ineffective. But without such soldering connection, the assembly of the members into the body would be much complicated since the strips and the PTC resistors, which are the separate members, must be inserted in the body at a time in such a way as to be correctly positioned with respect to each other and to the body. Accordingly, for satisfying easier assembling as well as stable and effective thermal contact between the PTC resistors and the electrodes, it is required to support the PTC resistor in correct position relative to the electrodes without using the solder and the like prior to assembling these members into the body or casing. However, there is no provision in the prior art for the above construction.